Prior to leasing this dedicated server, the IP addresses were assigned to someone else who was running an open SMTP (email) server. The entire scope of IP addresses were blacklisted on several spam block websites. I didn't realize this until after we set the server up and nobody could get our notification emails that the forum sends out to validate new accounts. We have worked hard to get our IP addresses removed from these lists and we have been successful, but AT&T and Verizon both have their own internal lists that are much harder to get removed from. Since we don't yet have a reverse lookup configured, AT&T and Verizon both still block emails originating from our IP scope.

Instead of paying to purchase reverse lookup from my ISP I am going to configure the forum software to utilize a remote SMTP server instead of our own. Not only will this resolve this issue but it also will free me of the burden of trying to keep an SMTP server secure from changing technologies. I'll keep you posted, but in the meantime know that this is why AT&T and Verizon customers cannot get forum related emails.